A 16-year-old boy was last night in custody at a north London police station after being arrested on suspicion of murdering a London teenager who was shot in the head in a suspected gang attack. Sharmaarke Hassan, 17, was shot at close range on Saturday night as he sat on a bench with friends in a Camden estate which is notorious for drug dealing.

Police have yet to establish a motive for the killing, although local Somali community leaders believe it is gang-related.

Superintendent Paul Morris, of Camden police, said: "This is a shocking tragedy. We've stepped up patrols in the borough to reassure the community and specialist officers are investigating the incident."

Detectives from Trident, the Metropolitan police unit tackling gun crime in the black community, have spoken to the local Somali community in an effort to defuse the threat of revenge attacks.

Yesterday it emerged that Hassan, the 15th teenager to die violently in London this year, was convicted last month of dealing cannabis and given a one-year community punishment order and 40 hours of community service.

The Crown Prosecution Service said he received an asbo in November last year, banning him from a street half a mile away from where he was shot. He broke it the next day and was referred to the youth offending panel.

He is understood to have been a member of a Somali gang, the Money Squad, said locally to have moved in on the lucrative drugs market around Camden and King's Cross.

Omar Yousef, a spokesman from the Camden Somali Forum, said tension had been rising for many months as rival gangs tussled over the drugs trade. "The consensus within the community is that this is the tragic end of a cycle of tit-for-tat violence. We are very concerned over what may happen next," he said.

Camden council and local police recently launched a blitz on drug dealing and gang activity in the area. Police and local councillors last night attended a community meeting to discuss the shooting.

Brian Coleman, London assembly member for Barnet and Camden, said: "I don't think the issue of gang-related crime among the Somali community, or any other community, in Camden is being taken seriously. We must do all we can to end this senseless violence. How long will it be before another ordinary person is caught up in the crossfire of these gangs?"

Hassan was not the first Somali, youth to die in the area. Mahir Osman, 18, died after being set upon by a gang of up to 40 Somali youths near Camden tube station in January 2006. A total of 13 youths and men - including the son of the former Ugandan dictator Idi Amin - were convicted at the Old Bailey last year of taking part in the attack.